Split the inv message reception processing in logical blocks by inv type

Summary:
This is a minor refactor that split the if/elseif/else block for block
and tx inv reception into 2 separated logical blocks. Each block holds
the cs_main lock locally so the scope is slightly reduced.

Depends on D9590.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D9591